In general, the pneumatic tires are provided in the tread portion with a plurality of main circumferential grooves extending circumferentially of the tire.
According to the recent remarkable improvement in vehicle performances, especially of the passenger cars, the running speed is increased, and a further improved drainage is required for wet performances such as wet grip performance, wet braking performance and the like. Therefore, to meet such requirement, a relatively wide groove having a straight configuration has became used for the circumferential groove.
In such a circumferential groove, as shown in FIG. 8, a trapezoidal cross sectional shape of which the top width WO is slightly wider than the bottom width, has been conventionally employed.
On the other hand, in the ground contacting patch of a tire, an air column is formed in the hollow space surrounded by the circumferential groove wall and the road surface.
In such a ground contacting patch, the main groove which has a conventional trapezoidal cross sectional shape, has a width being almost constant along its longitudinal direction as shown in FIG. 9. Accordingly, the air column is constant in the width and cross sectional shape and area. As a result, resonance is easily caused during running to increase the running noise.
Further, the air which is compressed between the tread surface and the road surface and then discharged through the circumferential groove, makes a so-called pumping noise. Due to the above-explained constant cross sectional area, the flow speed of the discharged air becomes increased in the front and rear edges of the ground contacting patch, which is a main cause of the pumping noise.
According to the use of a straight groove and the increase in the groove width, the air resonance noise and air pumping noise became remarkable.
If the groove width is decreased and a nonlinear groove is used, the noise may be reduced, but wet performances are deteriorated.